The invention relates to an optical record carrier comprising a recording layer having substantially parallel tracks for recording information in a pattern of optically detectable marks, the tracks being provided with wobbled grooves and predetermined positions at regular intervals along the tracks, part of the predetermined positions forming information positions on which information is stored in the form of pits.
When writing user information on a record carrier by means of a scanning radiation spot, it is in general desirable to know the position of the radiation spot on the record carrier. Since user information is not available on a virgin recordable record carrier to determine the position from, the position may be determined by reading position information stored in an embossed wobbled groove or embossed pits of the record carrier.
The recorded information is arranged in tracks. In general, a track is a line on the record carrier to be followed by a scanning device and which has a length of the order of a characteristic dimension of the record carrier. A track on a rectangular record carrier has a length substantially equal to the length or width of the record carrier. A track on a disc-shaped record carrier is a 360xc2x0 turn of a continuous spiral line or a circular line on the disc.
A track may comprise a groove and/or a land portion between grooves. A groove is a trench-like feature in a land portion of the recording layer, the bottom of the trench being nearer to or further away from the light-incident side of the record carrier. User information may be recorded on the lands and/or in the grooves in the form of optically detectable areas in the recording layer, e.g. as areas having a characteristic reflection or magnetization. The pits may be located on the lands or in the grooves.
A record carrier according to the preamble is known from European patent application no. 0 800 165, which discloses an optical record carrier having a groove, radially wobbled at a constant frequency without phase jumps and comprising pits formed at predetermined positions between turns of the wobbled groove. The pits are at predetermined positions where the wobble has a minimum or maximum deviation as measured from the centre of the pits. A disadvantage of this known record carrier is that the reliability of the detection of the pits reduces when user information is recorded in the tracks.
It is an object of the invention to provide a record carrier having a pit-groove structure which can be detected more reliably.
In accordance with the invention, the record carrier as described in the opening paragraph is characterised in that the phase of the wobble at the information positions is adapted to the presence of a pit at such a predetermined position, and the wobble changes phase between each two subsequent predetermined positions of which one of the positions contains no pit and the other position is an information position containing a pit. According to the invention, the phase of the wobble at a certain position along the track must be adapted to the presence or absence of a pit at such a position. The wobble of the groove enhances the signal obtained from the pits when scanning the record carrier. The detection margin of the pits will be substantially increased when the phase has a first value at a predetermined position where there is a pit and a second, different value at a predetermined position where there is no pit. The change in phase of the wobble between two consecutive positions, one without a pit and one with a pit, enhances the amplitude of a read signal from the record carrier. If an information position containing a pit is not preceded by another information position, the wobble must also show a phase change between the information position and the position preceding it, in order to achieve a proper detection margin also for the first information pit of a sequence of pits. The same applies to an information position followed by a position which is not an information position. The increased detection margin allows a reduction of the size of the pits, thereby reducing crosstalk from the pits on the signal representing user information recorded in the tracks. The increased margins also allow proper detection of the position information on a record carrier on which user information has been recorded.
The number of periods of the wobble on the record carrier having a phase pertaining to an information position containing no pit is preferably larger than the number of periods of the wobble having a different phase. Alternatively, the predetermined positions are preferably arranged in cells of substantially equal extent containing a number of periods of the wobble, the majority of the number of periods in substantially all cells having the same phase.
The equal phase of the majority of periods of the wobble facilitates synchronous detection of those periods of the wobble having a different phase. When the predetermined positions are arranged in cells, the first period of the wobble after the start of the cell pertains preferably to an information position having a pit. The pit and the associated phase of the wobble can be used as synchronisation mark, identifying the start of the cell. The detection margin of the pit will be increased if there is a phase change between the wobble and the last wobble of the preceding cell. The combination of a phase change in the wobble immediately followed by a pit further improves the recognition of the start of the cell. If several cells are grouped in sectors, the first cell of a sector has preferably a unique pattern of pits, e.g. a pit at each of the first two positions of the first cell, for synchronisation on the sector. The extent of a cell may be a linear extent, measured along a track and useful for a record carrier of the constant angular velocity (CLV) type, or it may be an angular extent, measured over an angular displacement of a circular system of tracks and useful for a record carrier of the constant angular velocity (CAV) type.
To enhance the read signal, the wobble has preferably a minimum value, as measured from the centre line connecting the pits along a track, at an information position with pit and a maximum value at an information position without pit. In other words, a minimum value of the wobble is a deviation of the groove in the direction of the pit. When the pits are located on a land portion between two neighbouring grooves, the two grooves are preferably in anti-phase. When the pits are depressions in the land and the grooves are trench-like, the information positions with pit are preferably located at positions where the land has a minimum width and the information positions without pit are preferably located at positions where the land has a maximum width.
The information positions may be arranged at equidistant positions along a track. They may also be grouped in series and the wobble in between subsequent series of information positions in the track direction may represent information. When the information in the wobble is encoded by phase-shift keying, the same phase variation of the wobble may be used for parts of the groove at the position of the predetermined positions and for parts of the groove in between the series of predetermined positions.
The predetermined positions are preferably grouped in doublets of two neighbouring predetermined positions, and only one pit is present in each doublet. A single logical value may be assigned to a doublet, e.g. a pit followed by a non-pit is a xe2x80x981xe2x80x99 and a non-pit followed by a pit is a xe2x80x980xe2x80x99. When the presence of a pit at both positions in a doublet is determined, the detection margin between a xe2x80x981xe2x80x99 and a xe2x80x980xe2x80x99 is increased compared to a detection at a single position.
In a record carrier wherein land portions are arranged between neighbouring grooves, the land portions are preferably alternatingly provided with pits and without pits.
When scanning a groove, there are pits on only one side of the groove, thereby reducing cross talk in the read signal between neighbouring tracks.
The objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which